It is often desirable to use an auto-exposure algorithm in a camera to ensure that the image is acquired at a balanced shutter-time and gain under different lighting conditions. Often the objective of the auto-exposure algorithm is to maintain a constant global luminance for the image by adjusting the shutter-time and gain. The problem with this approach is that if the scene contains objects that are very small (<1% of image pixels) but much brighter than the rest of the image, these objects will be over-exposed and appear saturated in the image. This problem is specifically severe for a camera equipped with an infrared (“IR”) illuminator because certain small objects of interest (e.g. license plates) have a much higher IR reflectivity than the rest of the scene. However if the camera ensures that no area of the image is saturated, a small background light in the image can result in the majority of the image being permanently underexposed.
There are several cameras in the market that try to solve the problems of saturated objects as a result of IR-Illumination by simply reducing the intensity of the IR illuminator. Controlling IR intensity to control saturation will only work for a camera with built-in IR illuminator which can be directly controlled. In particular, this approach will not work with cameras that use an external IR illuminator wherein the illumination intensity cannot be directly controlled.